Until relatively recently, standard keypads installed in coin operated telephones generated audio signals directly onto the telephone lines. The keypads also reflected an impedance to balance the line and consequently pulled roughly 10 milliamperes (mA) during operation.
The deregulation of the telephone industry and subsequent new telephone standards created a demand for low power controllers having many features for use in coin operated telephones. These low power controllers overcame stringent power constraints by utilizing various low power circuits and power management schemes.
In one highly advanced low power electronic controller, the LES 100 WE Retrofit Kit, developed and sold by Mars Electronics, the power constraints on the controller did not permit the usage of 10 mA for use with the keypad.
Consequently, the keypads were modified to remove that line balancing capability and therefore reduce their current requirements before being connected to an audio circuit in the controller. The keypad still generated audio signals, but these signals were processed and then balanced to the line by the audio circuit, thus enabling the keypad to draw much less current when operating.
In an alternative approach, circuitry was provided to disable the keypad after a half-second of dual-tone generation by a key. This limited the power drawn by the keypad. For commercial reasons, this approach has not been accepted in the market place. For example, such approach cannot be used where it is desired to access the messages on an answering machine which is typically designed to require a 2 to 3 second tone duration to achieve access.
A minimum of four Plain Old Telephone Service (POTS) input lines were required for the low power controller keypad implementation. The input lines were used to indicate key pushed, power in, disable and return to ground.
While the Mars Electronics LES 100 has enjoyed immediate commercial success as a retrofit controller for use with standard Western Electric payphones, its installation has required the modification of every keyboard to remove its line balancing components before connection to the controller. Because all payphone keypads need modification for use with this controller, and because subsequent improvements to the controller may render older controllers employed with modified keypads obsolete, it was highly desirable to design a single controller readily suitable for use with either a standard, a modified keypad, or some other alternative keying arrangement.